From: Remi Pommarel <repk@triplefau.lt>
To: Eric Anholt <eric@anholt.net>
Cc: Stephen Warren <swarren@wwwdotorg.org>,
Lee Jones <lee@kernel.org>,
Michael Turquette <mturquette@baylibre.com>,
Stephen Boyd <sboyd@codeaurora.org>,
linux-rpi-kernel@lists.infradead.org, linux-clk@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor
Date: Wed, 2 Dec 2015 23:21:52 +0100 [thread overview]
Message-ID: <20151202222152.GH12775@cruxbox> (raw)
In-Reply-To: <20151118191141.GB491@cruxbox>
Hi Eric
On Wed, Nov 18, 2015 at 07:11:41PM -0100, Remi Pommarel wrote:
> On Wed, Nov 18, 2015 at 10:25:45AM -0800, Eric Anholt wrote:
> > Remi Pommarel <repk@triplefau.lt> writes:
> >
> > > Make bcm2835_clock_choose_div always round up the chosen MASH divisor so that
> > > the resulting average rate will not be higher than the requested one.
> > >
> > > Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> > > ---
> > > drivers/clk/bcm/clk-bcm2835.c | 15 ++++++++-------
> > > 1 file changed, 8 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> > > index 39bf582..1237716 100644
> > > --- a/drivers/clk/bcm/clk-bcm2835.c
> > > +++ b/drivers/clk/bcm/clk-bcm2835.c
> > > @@ -1152,18 +1152,19 @@ static u32 bcm2835_clock_choose_div(struct clk_hw *hw,
> > > {
> > > struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> > > const struct bcm2835_clock_data *data = clock->data;
> > > - u32 unused_frac_mask = GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0);
> > > + u32 unused_frac_mask =
> > > + GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0) >> 1;
> > > u64 temp = (u64)parent_rate << CM_DIV_FRAC_BITS;
> > > + u64 rem;
> > > u32 div;
> > >
> > > - do_div(temp, rate);
> > > + rem = do_div(temp, rate);
> > > div = temp;
> > >
> > > - /* Round and mask off the unused bits */
> > > - if (unused_frac_mask != 0) {
> > > - div += unused_frac_mask >> 1;
> > > - div &= ~unused_frac_mask;
> > > - }
> > > + /* Round up and mask off the unused bits */
> > > + if ((div & unused_frac_mask) != 0 || rem != 0)
> > > + div += unused_frac_mask + 1;
> > > + div &= ~unused_frac_mask;
> >
> > Suppose we've got 8 of our 12 frac bits populated. You've added a ">>
> > 1" to the unused_frac_mask, so it's only 0x7 instead of 0xf. When you
> > say "round up", you add 0x8 (the high bit of the unused mask") then and
> > with ~0x7. If you started with 0x1 in the low bits of div, you'd end up
> > with 0x8, so you've set an unused bit instead of actually rounding up.
> >
> > Did my logic work, here? I think you just want to drop the ">>1" in
> > unused_frac_mask.
>
> I don't think so.
>
> If we have 8 of our 12 frac bits populated GENMASK(12 - 8, 0) will be 0x1f
> because GENMASK(4, 0) generates a mask from bit at position 0 to bit at
> position 4 inclusively (which is the fifth bit). So GENMASK(4, 0) >> 1 will
> be 0xf which is what we want here.
Does my logic seems sensible to you or am I missing something ? I just
want to be sure we agree before I send another version for this
patchset.
Thanks.
Best Regards
--
Remi
next prev parent reply other threads:[~2015-12-02 22:21 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-11 14:22 [PATCH v2 0/3] Add PWM clock support for bcm2835 Remi Pommarel
2015-11-11 14:22 ` [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor Remi Pommarel
2015-11-18 18:25 ` Eric Anholt
2015-11-18 19:11 ` Remi Pommarel
2015-12-02 22:21 ` Remi Pommarel [this message]
2015-12-04 0:31 ` Eric Anholt
2015-11-11 14:22 ` [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection Remi Pommarel
2015-11-18 18:30 ` Eric Anholt
2015-11-18 19:24 ` Remi Pommarel
2015-12-04 0:37 ` Eric Anholt
2015-12-04 20:37 ` Remi Pommarel
2015-12-06 0:19 ` Eric Anholt
2015-11-11 14:22 ` [PATCH v2 3/3] clk: bcm2835: Add PWM clock support Remi Pommarel
2015-11-18 18:32 ` Eric Anholt
2015-11-18 19:26 ` Remi Pommarel
2015-11-28 20:52 ` [PATCH v2 0/3] Add PWM clock support for bcm2835 Stefan Wahren
2015-11-29 0:31 ` Remi Pommarel
2015-11-29 21:22 ` Stefan Wahren
2015-11-29 22:25 ` Remi Pommarel
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